FIG. 1 is a schematic diagram of a configuration of a conventional in-chip transformer. The conventional transformer 10 comprises a primary coil 120 and a secondary coil 140. The primary coil 120 has two end points P1 and P2, and the secondary coil 140 also has two end points S1 and S2. In this example, the transformer 10 is a planar transformer, i.e., the primary coil 120 and the secondary coil 140 are planar coils and are on different planes, e.g., the primary coil 120 is right above or under the secondary coil 140. The transformer 10 can be employed as a balun. In the following description, the end point S1 of the secondary coil 140 is assumed to be coupled to ground, as an example.
Because the end point S1 of the secondary coil 140 is coupled to ground while the end point S2 is not coupled to ground, the end points S1 and S2 have different impedances. Because the transformer 10 is a planar transformer, and the points P1′ and P2′ are respectively right above or under the end points P1 and P2, the two end points P1 and P2 respectively correspond to two points P1′ and P2′ of the secondary coil 140. Referring to FIG. 1, the corresponding point P1′ has a longer distance from the end point S2 compared to the distance between the corresponding point P2 and the end point S2. That is, signal transmission distances between the point P1′ and the end point S2 and that between the corresponding point P2′ and the end point S2 are not equal. Since the end points S1 and S2 have different impedance values, and the signal transmission distances between the corresponding point P1′ and the end point S2 and that of the corresponding point P2′ and the end point S2 are not equal, the two end points P1 and P2 of the secondary coil 120 respectively have different input impedances. Therefore, when input signals having equal energy are respectively inputted to the two end points P1 and P2 of the primary coil 120, two output signals have unequal energy at the end point S2 of the secondary coil 140, thereby creating a problem of unequal energy of the output signals of the transformer.
In addition, when the transformer 10 is used in a transmitter of a communication system, wherein circuits of the transmitter are non-ideal, on top of a to-be-transmitted signal, second-order harmonic signals of the to-be-transmitted signal are transmitted. When signal strength (energy) of the to-be-transmitted signal of the transmitter becomes larger, the signal strengths of the second-order harmonic signals become larger. Large second-order harmonic signals will cause interference to a circuit having an on-chip inductor, such as a voltage-controlled oscillator (VCO) where its output frequency may have undesired shift because of the interference; however, the conventional transformer described above is unable to remove the undesired signals.
Therefore, a transformer capable of outputting output signals having equal output signal strength and removing undesired signals (e.g., second-order harmonic signals) is in need.